One I did in similar situation, which went down very well, was playing 'Decimal Duck' with objective to do with place value. You need large digit cards and in place of a decimal point, a duck. Children hold cards up to give a number, and another child multiplies or divides number by 10, 100 or 1000 by moving children, but keeping duck still. Can include individual work with whiteboards, and children reading out numbers etc. Hope this helps.

im a trainee on my last placement and use this with my yr 5 class all the time and based it on topics we have studied. they love it as they never get to use the whiteboard. its an interactive whiteboard resource based on 'who wants to be a millionnaire'. i got the blank one off primaryresources and put in my own questions. the children use whiteboards to show there answers. then you can go through the ways to work out the questions for the chn who got them wrong. i hope this helps. i can send you mine by email if you want.

We interviewed for DH post last week and this is what the 'winner' did!

Gave the children a x, /, -, + card and asked them to hold them up at the right point. He put a powerpoint up showing different mathematical language. He made them show at the same time so no cheating. He then taught them karate signs which was hilarious and all loved it.

To move them on, he then continued on to word problems and they simply had to show the correct sign and even included some multistep ones for the more able.

I enjoyed the 20 minutes and so did the children. I think he could have given them a little more discussion time for the more difficult ones as the less able were a little lost.

Ask them to close their eyes and imagine two buses- a red one and a green one(any colour will do). Then say 10 children get on the red bus and 10 children get on the green bus, then, 5 children get off the green bus and on to the red bus, one child gets off the red bus and on to the green bus. How many on the red/green bus? You can make it harder or simpler as you go. They love it! Make sure you have written down what you've said. practise with a class before the interview.
(I like the sound of the karate too.)
Good luck.

Before i came back on to TES to read your ideas this is what a friend suggested.

start by chanting x3 table (using different voices to make it fun)
then count on from a two digit number in 3's

Then tell them that im going to teach them how to decide if a number (a 2, 3, 4, or 5 digit) is divisible by 3.

using the splat square (100 square on internet site that lets u splat any number) choose three numbers and tell them that if we choose for example 5, 4 and 3 to make the number 543. then to deide if it can be divided by three all we have to do is add up all three digits
5+4+3 = 12
12 is in 3 times table therefore 543 can be divided by 3.
children can use whitboards and then lead off into short partner work making numbers for them and getting them to check each others.

what my friend has told me is easy and approachable but im not sure if its going to be a mental maths starter that has got a 'wow' factor to get me the job.

i like the ideas above suggested but feel like im now stuck with my mind buzzing

I like the idea of times tables in different voices, but did it once in a placement lesson and the teacher completely slated me for it - te chn were unbelievably well behaved at that school and he seemed very adverse to anything that got them a little bit giddy - they weren't particularly giddy, just enjoying themselves - anyway, just be aware that some people dont like stuff like that! Good luck!